The present invention relates to a read system for reading information from a magnetic storage medium using a magnetoresistive head and for providing an output signal representative of the information read. In particular, the present invention relates to a read system employing a negative feedback impedance matched preamplifier for improved bandwidth and noise performance.
A popular method of magnetic data storage utilizes magnetoresistive (MR) heads to recover data from a magnetic data storage medium such as a magnetic disk. An MR head employs an MR element that changes in resistivity with changing magnetic flux from data patterns on an adjacent magnetic disk surface. A bias current having a constant value is passed through the MR element, or a bias voltage having a constant value is applied across the MR element, and the change in resistivity is measured by sensing a change in voltage across the MR head. The change in voltage is detected by the preamplifier circuit for manipulation and analysis, so that the data recorded on the disk may be accurately recovered.
Preamplifier circuits have certain frequency response and noise performance characteristics. These characteristics and others determine the usefulness of the amplifier circuit in any given application. The band of frequencies over which the gain of the amplifier circuit is almost constant is called the bandwidth. Signals whose frequencies are outside the bandwidth will experience lower gain, with the gain decreasing as the frequencies of the signals become further removed from the bandwidth. The preamplifier circuit is only effective for input signal frequencies within its bandwidth, since input signals having a range of frequencies outside of the preamplifier bandwidth would be distorted, with different frequency components of the input signal being amplified by different amounts. As data continues to be recorded at higher and higher frequencies in order to store a greater amount of data on a fixed medium, higher frequency bandwidth is desirable.
Noise performance can be a limiting factor on the effective bandwidth of a preamplifier circuit. The preamplifier circuit is typically connected to the MR head through a flex cable, which acts as a simple transmission line. At high frequencies, the transmission line can cause signal reflections that distort the output signal and reduce the signal-to-noise ratio (SNR) of the preamplifier circuit. These reflections can be reduced or eliminated by either impedance matching the MR head to the transmission line and/or by impedance matching the preamplifier circuit to the transmission line. Since the resistance of the MR head can vary significantly between head designs, it is not feasible to precisely match the impedance of the MR head to the impedance of the transmission line. Thus, the practical approach for noise reduction and improved bandwidth is to match the impedance of the preamplifier circuit to the impedance of the transmission line.
Prior attempts to match the impedance of the preamplifier circuit to the impedance of the transmission line have involved employing a shunt matching resistor located across the input nodes of the preamplifier circuit, effectively in parallel with the input impedance of the preamplifier circuit. This configuration allows the characteristic impedance of the preamplifier circuit to be reduced to match the impedance of the transmission line. As a result of this impedance matching, the high frequency reflections that could distort the output signal from the preamplifier circuit are reduced, therefore increasing the undistorted bandwidth of the preamplifier circuit. However, there is a large passband noise penalty incurred by the use of the shunt matching resistor, which may not be acceptable for high performance disk drives or other applications.
There is a need in the art for a preamplifier circuit that is impedance matched to the transmission line connecting the preamplifier circuit to the MR head in such a manner as to improve the bandwidth of the preamplifier circuit while maintaining acceptable noise performance.
The present invention is an impedance matched preamplifier system for connection through an interconnect to a read head. The interconnect has a characteristic impedance associated therewith. The preamplifier system includes an amplifier circuit having an input for connection to the interconnect (or to the head, in one embodiment). The amplifier circuit amplifies an input signal carried from the read head through the interconnect, yielding an amplified output signal. A feedback resistance is connected between the amplified output signal and the input of the amplifier circuit. The feedback resistance has a value selected to provide an effective input impedance of the preamplifier system to match the characteristic impedance of the interconnect.